The present invention relates generally to methods and apparatus for using eddy current probes to non-destructively inspect and detect the presence of hidden fatigue cracks located underneath the head of a raised head fastener, such as raised head rivets used in joining aluminum skins of aircraft fuselages or bridge assembly girders.
Eddy Current (EC) inspection techniques use the principles of electromagnetic induction to identify or differentiate changes in structural conditions in conductive materials (e.g., metals). The presence of a crack is indicated by localized changes or perturbations in the flow of eddy currents in the conductive material (e.g., the aluminum skin of a helicopter) induced by the probe. Eddy current signals from EC probes are typically monitored using impedance-plane plots, which show the reactive (amplitude) component along the X-axis and resistive (phase) component along the Y-axis of an alternating electric current flowing through a coil of wire (i.e., magnetic test coil) housed inside of the EC probe. An alternating electric current flowing in the test coil creates an alternating magnetic field, which, when placed close enough to the conductive material, induces eddy currents to flow on the surface, and inside of, the conductive material. These induced eddy currents, in turn, generate their own magnetic field in a direction that opposes the applied magnetic field of the test coil. This opposing magnetic field changes the impedance of the test coil in a way that can be measured and displayed.
During EC inspection, the presence of a crack (or a change in the thickness of a part, change in conductivity, or other type of material discontinuity) causes localized perturbations in the flowing eddy currents induced by the probe. These perturbations show up as identifiable changes in the impedance-plane plot of the probe's test coil. EC instruments record these impedance changes, and display them in impedance-plane plots to aid in the flaw detection process.
Because eddy currents are created using an electromagnetic induction technique, the inspection method does not require direct electrical contact with the part being inspected. Also, the depth of penetration of eddy currents is inversely proportional to the product of three factors: magnetic permeability, electrical conductivity, and frequency of the alternating inducing currents. Therefore, eddy current tests are most sensitive to discontinuities on the surface underneath the coil, which makes them very effective for detecting fatigue cracks in the surface or near-surface region. Typically, high frequency eddy currents (HFEC) are generally considered to be 100 kHz and above and are used to detect near-surface flaws. Low frequency eddy currents (LFEC) are in the range of 100 Hz to 10 kHz and are used to penetrate deeper to detect flaws in the underlying structure. The thicker the structure, the lower the EC operating frequency that is required to inspect it. However, eddy currents flowing deeper in the material are weaker, and lag in phase, compared to the eddy currents near the surface or at an inner wall.
Eddy currents induced by an EC probe test coil are not uniformly distributed throughout the conductive skin. Rather, they are densest and strongest at the skin's surface immediately beneath the coil, and become progressively less dense (i.e., weaker) with increasing distance below the surface; as well as with increasing radial distance away from the coil's centerline. Thus, the inspection sensitivity (i.e., flaw detection sensitivity) is decreased by a “lift-off” effect as the gap between the probe head and the surface being inspected increases. This loss in sensitivity caused by the “lift-off” effect is significant when the EC probe is placed on top of the head of a raised head fastener, especially when trying to detect cracks in the structural member (i.e., skin) directly underneath the fastener's head (a typical raised head rivet can have a lift-off distance of as much as 1-3 mm). Hence, it is desirable to minimize the gap or “lift-off” distance between the EC probe's head and the skin being inspected.
Raised head fasteners (e.g., rivets) that are typically found in rotorcraft joints, aircraft fuselage frames, and many civilian structures (ships, bridges, towers, etc), interfere with the ability to place conventional eddy current probes close to the area of interest; i.e., the structural skin underneath the raised head fastener. It is difficult, if not impossible, with current technology (i.e., conventional EC probes) to generate sufficient eddy current fields under the fastener head for crack detection. Only when the fatigue crack has radially extended far enough outwards from underneath the fastener head (i.e., beyond the rivet head's outer radius, where it becomes visible), can conventional hand-held EC probes be successfully used.
When using any hand-held EC probes on raised head fasteners it is difficult to provide a consistent and stable orientation of the test coil with respect to the radial and circumferential position (relative to the rivet's centerline). It is also difficult to maintain the orientation of the coil's axis perpendicular to the rivet head's surface. Uncontrollable changes in the probes position and orientation as the probe is manually moved around on the rivet head and on the skin next to the rivet head, degrades the reliability and repeatability of the eddy current signal. These inspection impediments make crack detection around raised-head fasteners impossible to achieve with hand-held EC probes (such as pencil-type hand-held probes).
The small size of the test coil, and associated high operating frequencies, in conventional pencil-type EC probes limits the ability to detect cracks in the second, lower layer of skin. This is because lower EC frequencies are required to inspect the deeper regions of the structure, which, in turn, require larger diameter magnetic coils. Pencil-type EC probes also have difficulty detecting hidden, angled cracks underneath rivet heads (i.e., cracks that have not penetrated to the upper surface of the skin).
Other factors also affect the repeatability, reliability, and accuracy of crack detection using EC probes. In particular, a common problem involves the degree of electrical conductivity between the rivet fastener, the surface and the subsurface layers. The level of conductivity affects the eddy current signal by affecting the amount of current flowing through the rivet (as opposed to flowing through the skin). Many factors can affect the conductivity levels, e.g. (1) the type of fastener coating (e.g., an alodine coating on an aluminum rivet is less conductive than an anodized coating), and (2) the degree of coating removal from the fastener caused by fretting of the mating surfaces, which typically increases the joint conductivity; and (3) corrosion at the mating surfaces. Variations in the conductivity levels, and associated changes in eddy current response, can be a significant impediment to obtaining successful inspections. Small changes in the EC signal produced by cracks at rivet sites may be lost or overwhelmed by larger changes in the EC signal caused by joints with higher conductivity. Hence, a need exists for a way to eliminate uncertainties and “noise floor” problems caused by high conductivity joints.
FIG. 1A shows a schematic cross-section side view of a raised head fastener 2 (e.g., a solid rivet) holding together two structural skins, upper skin 7 and lower skin 8 (e.g., 2024 or 7075 aluminum alloy plates). Rivet 2 has a raised head 3, a cylindrical shank 4, and a deformed button (rivet upset region) 5 on the bottom. The rivet's shank 4 is located inside hole 6 in the structural skin 7 & 8. Fatigue crack 9 in upper skin 7 is shown lying underneath the fastener's head 3, completely hidden from view. In this example, crack 9 is illustrated as being located in the upper structural skin 7. However, such cracks can further extend into the lower skin 8, or, be located only in the lower skin, etc. Multiple fatigue cracks may emanate radially from hole 6 at different circumferential locations (e.g., at 0 and 180 degrees), depending on the stress state. These cracks may be thru-cracks (penetrating completely through a structural layer (i.e. skin) from front-to-back); or they may be sub-surface cracks that don't completely penetrate through the skin; and/or they may penetrate just one surface of a skin (e.g., an upper or a lower surface of a skin).
FIG. 1B shows a pair of conventional, hand-held eddy current probes 101 & 103 (e.g., “pencil” or “spot” probes). Probe 101 (with internal test coil 102) is manually placed directly on top of the rivet head 3, concentric with the rivet's centerline (i.e., axis of rotation). When probe 101 is held at this location, successful detection of hidden crack 9 can be difficult because of the large lift-off distance (i.e., the thickness, thead, of rivet head 3), which reduces the amount of eddy current field 105 that can interact with crack 9. Alternatively, pencil probe 103 can be placed in direct contact with the upper/exterior surface of upper skin 7, outside of the rivet head's outer radius. However, even though the lift-off distance is essentially zero at this location, the concentrated eddy current field 106 is still located far away from crack 9 and, hence, can limit successful detection of crack 9 when hidden underneath rivet head 3.
In FIG. 1C, a conventional EC probe 101 is hand-held in contact with the rivet's head 3, inclined at an inclined angle, α, with respect to the rivet's centerline, and located at a radial distance, Rprobe. As the inspector moves probe 101 by hand around the surface of the rivet, scanning and searching for cracks, the probe's radial position, Rprobe, the inclination angle, α, and the gap/lift-off distance from the rivet head's surface, cannot be held perfectly steady/constant. These unavoidable changes in the probe's position, despite the best efforts of an experienced inspector, adds a large amount of undesirable “noise” to the probe's output signal that is actually in excess of the response generated by the fatigue crack itself. This eliminates the possibility of successful crack detection.
Because the use of hand-held EC probes is typically a labor-intensive, manual activity, a need exists for: 1) improved methods and probe designs that minimize the time needed to inspect a riveted structure, 2) a probe design that eliminates the position variations, and associated noise, to produce successful crack detection beneath raised-head fasteners, and 3) minimizing the need to go back and perform repeated inspections due to inconsistent or unreliable readings.
What is needed is an improved EC probe design that provides reliable, repeatable, accurate and consistent readings by eliminating the geometrical uncertainties associated with hand-held, manual positioning.